1. Field of the Invention
The present invention relates to an incoming current suppression device, particularly to the incoming current suppression device which suppresses an exciting incoming current flowing from a three-phase AC power supply into a three-phase transformer through a three-phase breaker.
2. Description of the Background Art
Conventionally it is known that a transient current, which refers to an exciting incoming current, flows from the three-phase AC power supply to the three-phase transformer when throwing the three-phase breaker. The exciting incoming current becomes several times to ten times larger than the rated current of the three-phase transformer and causes instantaneous drop of the three-phase alternating voltage and the like.
As a method of suppressing the exciting incoming current, a method is known where a residual magnetic flux of each phase of the three-phase transformer is detected, and the alternating voltage of a reference phase is thrown at a timing when a stationary magnetic flux and the residual magnetic flux of the reference phase are coincident with each other, followed by throwing the alternating voltages of two remaining phases at the timing when the alternating voltage of the reference phase becomes 0 (for example, see Japanese Patent Laying-Open Nos. 2004-208394 and 2006-040566.)
In the conventional incoming current suppression method, it is a precondition that the stationary magnetic flux and the residual magnetic flux of each remaining phase are coincident with each other at the timing when the alternating voltage of the reference phase becomes 0.
However, in a case where an interpole capacitor is connected in parallel to each of three switches included in the three-phase breaker, the three-phase alternating voltage is supplied to the three-phase transformer through the three interpole capacitors even if the three switches are opened. For this reason, the residual magnetic flux of each phase of the three-phase transformer is the magnetic flux of a direct-current component on which the magnetic flux of a commercial frequency component is superposed.
Therefore, the residual magnetic fluxes of the two remaining phases are not always substantially the same at the time of throwing the reference phase, even if the direct-current components of the residual magnetic fluxes of the two remaining phases are substantially the same. If the two remaining phases are thrown at the above-mentioned timing under the state where the residual magnetic fluxes of the two remaining phases have some difference, the stationary magnetic flux and the residual phase of the two remaining phases will not be coincident with each other and thus a large exciting incoming current will flow.